This invention relates to an electronic flash apparatus for a photographic purpose and more particularly to improvements on the electronic flash apparatus which is provided with a termination circuit for terminating a discharge circuit.
Already known is an electronic flash apparatus which is provided with a termination circuit for terminating a discharge circuit and can emit a desired amount of light by restricting the period of time for which a flash tube is allowed to give forth light. There will now be briefly described the prior art electronic flash apparatus by reference to the circuit diagram of FIG. 1.
The electronic flash apparatus of FIG. 1 comprises a D.C. power source 2, a discharge circuit 4 for discharging electric energy supplied by the D.c. source 2 and a termination circuit 6 for terminating a discharge by the discharge circuit 4. The discharge circuit 4 comprises an energy storage capacitor 7 connected between the positive and negative terminals of the D.C. power source 2; and a series circuit formed of a flash tube 8 and a first silicon controlled rectifier (hereinafter abbreviated as "SCR") 10 and connected in parallel to the energy storage capacitor 7. When the SCR 10 and flash tube 8 are energized, and discharge circuit 4 discharges electric energy stored in the energy storage capacitor 7 charged by the D.C. power source 2, thereby emitting a flash from the flash tube 8.
The flash tube 8 is energized when its trigger electrode is supplied with high voltage pulses emitted by a high voltage pulse generator 14 interlocking with the operation of a control circuit 12. Like the flash tube 8, the first SCR 10 is energized when its gate is impressed with a trigger voltage produced by a trigger circuit 13 interlocking with the control circuit 12. The control circuit 12 is generally included in the camera shutter and operated by a switch 11 which is closed interlockingly with the movement of a shutter blade or blind.
With termination circuit 6 for terminating the discharge circuit 4, a series circuit of a commutation capacitor 16 and a second SCR 18 jointly constituting a reverse voltage passage is connected in parallel to the first SCR 10. To form a charge passage for the commutation capacitor 16, a resistor 20 is connected between the common junction of one end of the commutation capacitor 16 and the second SCR 18 on one hand and the positive terminal of the D.C. power source 2 on the other. Further, a resistor 22 is connected between the other end of the commutation capacitor 16, and the negative terminal of the D.C. power source 2 on the other. The termination circuit 6 energized the second SCR 18 to cause energy stored in the commutation capacitor 16 to be discharged, thereby temporarily applying a reverse voltage across the anode and cathode of the first SCR 10 and in consequence rendering the first SCR 10 nonconducting. When the first SCR 10 becomes inoperative, then the discharge circuit 4 is cut off. Later, the flash tube 8 terminates to emit flash. The second SCR 18 is energized by a trigger circuit 26 which produces a trigger voltage upon receipt of a signal from a timer or photoelectric converter 24. However, the prior art electronic flash apparatus of the above-mentioned arrangement has the drawback that since the commutation capacitor 16 takes much time to be charged again, the electronic flash apparatus needs a long time to be made ready for each succeeding emission of flash through the charge of the commutation capacitor 16. Generally, even when the first SCR 10 is rendered nonconducting, the impedance of the flash tube 8 does not immediately increase, but causes discharge current to continue to flow through the flash tube 8 for a short time. As the result, the commutation capacitor 16 is charged through a path formed of the flash tube 8, the commutation capacitor 16 and second SCR 18. When the commutation capacitor 16 is charged through the flash tube 8, then the charge takes place with a different polarity from the case where charging is normally carried out through the resistors 20, 22, namely, with a reverse polarity as shown by the + and - notations enclosed in parentheses. Where, therefore, the commutation capacitor 16 should be charged through the normal path, namely, through the resistors 20, 22, it is necessary to continue charging until the polarity is charged from the reverse to the normal operative form, thus requiring much time for storage of energy in the commutation capacitor 16.